This invention relates generally to down-the-hole, percussive drills, and more particularly to retention devices for retaining a head section of a bit should the head section separate from a shank of the bit.
In percussion downhole drilling, broken bits are a common occurrence because of the severe impact and rotational loads necessary to accomplish the drilling effect. Failures in the shank result in the head portion of the bit remaining in the hole. Time consuming and expensive procedures are required to remove the head before drilling can continue. If the head cannot be removed, the hole must be abandoned.
It is common practice to provide external "fishing" threads on the rear portion of the bit head to facilitate removal of the broken bit head by use of an internally threaded "fishing" tool. This is a somewhat difficult separate operation and results are very often futile.
Therefore it would be very advantageous to provide a system where the broken portion of the bit is automatically retrieved with the drill string and hammer.
One device for this purpose as described in U.S. Pat. No. 5,065,827 (Meyers, et. al.). This device includes an attachment piece with internal threads and a corresponding external thread on the bit. Such devices have several disadvantages. The threaded engagement may be lost if reverse rotation occurs after bit failure, resulting in bit head loss. Meyers suggests that a pin may be installed to prevent this, but this complicates removal even more. Also, if the threads are damaged in normal drilling, the removal of an unbroken bit for sharpening or other service is difficult or impossible without destroying the device.
The foregoing illustrates limitations known to exist in present drill bit retention devices. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above.
Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.